Electrical components used for industrial, commercial, or residential applications require enclosures to meet applicable code requirements, improve safety by preventing insertion of foreign objects, and slow deterioration of the components caused by dust, oils, and other elements. Electrical enclosures must be designed to provide the necessary protection for the environment surrounding the panel, while minimizing occupied space to meet dimensional constraints and reduce manufacturing costs.
As devices built from power semiconductor technology, such as insulated gate bipolar transistors (IGBTs) and silicon-controlled rectifiers (SCRs) have decreased in cost and improved in reliability, devices such as variable frequency drives (VFDs), motor soft starters, and other power conversion equipment are more commonly used to control motors. One of the biggest challenges in designing electrical enclosures for power conversion equipment is providing a means for heat dissipation. Semiconductor devices produce heat from switching losses and parasitic impedances inherent in their manufacturing. Other required devices, such as reactors and filters introduce voltage drops in the circuit and dissipate the energy as heat.
Electrical enclosures for power conversion equipment sometimes utilize forced convection from strategically located fans or other blower units to solve heating problems and maintain safe operating temperatures. While forced convection techniques provide effective cooling, the added components increase manufacturing costs, and create additional maintenance expenses as fans have a limited life expectancy. Consequently, natural convection cooling is often utilized when possible. Effective cooling with natural convection requires proper mechanical design to ensure adequate airflow and heat exchange.
FIG. 1 illustrates a common electrical enclosure assembly 101 with a cover 102 fastened to the sides 103 of the assembly 101. The cover 102 of the enclosure assembly 101 contains louvers 105, which are a series of narrow ventilation openings with overlapping slats. It is common practice in the art to form such louvers 105 in order to permit heat exchange, while providing a degree of protection against falling dust particles and entry of other foreign material. Common design practice includes louvers in both the bottom and top portions of the cover for intake of cooler ambient air and discharge of warmer interior air, respectively, along the plane. The flat top surface 106 of the enclosure assembly 101 typically will not contain ventilation openings. FIG. 2 shows an exploded view of the louver 105 contained within the bracket (2 in FIG. 1).
The louvers 105 allow for the entry of fresh, cooler air into the front of the enclosure's interior 107, lowering the temperature of the air in contact with electrical components contained within the enclosure assembly 101. The temperature of this air contained within the enclosure assembly 101 and in contact with components contained therein is commonly referred to in the art as the internal ambient temperature.
FIG. 3 illustrates a rear view of the enclosure assembly 101. In this depiction, the cover 102 is partially open. The inner side surface 108 forms an abutment surface with the right side surface 103 of the enclosure assembly 101. Electrical components (not shown) are mounted on the interior of the back panel 109.
FIG. 4 illustrates the rear view of the cover 102, including the inner side surface 108 with the louvers 105.
Another common design practice is to increase the enclosure assembly dimensions to increase surface area. One well-known technique of increasing surface area is to include fins on the enclosure cover or sides, as disclosed in U.S. Pat. Nos. 6,201,700; 6,628,521; and 6,979,772. This design practice often increases the overall quantity of material required for manufacture, while reducing the increases in overall dimensions. Those skilled in the art will recognize that a larger surface area, achieved by the addition of fins or expanded height, width, or depth dimensions of the enclosure, increases the amount of its radiated heat energy. This increase in heat energy radiated from the enclosure increases the overall heat dissipation of the enclosure, and therefore, lowers the internal ambient temperature.
One skilled in the art will recognize that the steady-state temperature of electrical components at rated operating conditions can be determined by adding the component temperature rise at rated conditions to the ambient temperature. In the case of components mounted within an electrical enclosure assembly, the internal ambient temperature, rather than the temperature of the air outside the enclosure, determines the component steady-state operating temperature. Therefore, the enclosure design has a critical influence on the steady-state operating temperature of all components contained within the enclosure assembly.
Each of these design practices utilized in prior art decrease the internal ambient temperature and improves cooling of components, but they also have disadvantages. Louvers are difficult and costly to manufacture. In the case of a metal enclosure, the manufacturing method would need to include a stamping process. With polymer enclosures, the molding process becomes more difficult, or an additional milling step is required. With both enclosure types, manufacturing costs increase and production rates decrease. Louvers also do not provide complete protection against insertion of foreign objects into the enclosure assembly. For this reason, a separate screen or filter is often required behind the louvers on the inside of the cover. Adding fins to the cover or sides also increases the quantity of material required, and makes the installation of a control panel or conduit entry points on the enclosure very difficult, driving up overall manufacturing and installation costs. Increasing dimensions of the enclosure assembly increases manufacturing costs, as more material is required, and also requires more space for mounting. As the cost of real estate increases, commercial and residential builders seek to maximize useable office and living space, while minimizing the space occupied by plumbing and control rooms. Reducing the dimensions of electrical enclosure assemblies allows for less occupied wall or floor space and, therefore, smaller electrical control rooms in commercial and residential buildings. Eliminating the need for louvers or fins reduces the quantity of material required, decreases costs, and improves manufacturing efficiency.
The present invention is directed to improvements in cooling of electrical enclosure assemblies.